Organic electroluminescent devices can be classified into low molecular weight organic electroluminescent devices and polymer organic electroluminescent devices depending on the materials and manufacturing process used to produce the organic electroluminescent device. Low molecular weight molecules can be easily synthesized and emissive compounds for red, green and blue light, which are the three primary colors of a visible range, can be synthesized by obtaining the proper molecular structures.
There are advantages to manufacturing low molecular weight organic electroluminescent devices. Since thin films may be formed by vacuum deposition, emissive materials may be easily refined and purified to a higher degree and color pixels may more easily be achieved. In order to apply the low molecular weight organic electroluminescent devices to practical applications, however, an improvement in quantum efficiency and color purity is needed and the crystallization of thin films must be prevented. Low molecular weight electroluminescent devices are now commercially applied to small-sized panels for mobile communication and car radios.
Research on polymer electroluminescent devices has accelerated since the discovery of the ability of poly(1,4-phenylene vinylene) (PPV), a π-conjugated polymer, to emit light when exposed to electricity. π-conjugated polymers have an alternating structure of single bonds (σ-bonds) and double bonds (π-bonds), where π-electrons are evenly distributed and free to move in the polymer chain. With proper molecular designing, π-conjugated polymers may have semiconducting properties and may emit light in a visible range corresponding to the HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) energy bandgap when included in an electroluminescent layer of an electroluminescent device. Furthermore, such a polymer can easily be formed into a thin layer in electroluminescent devices by spin coating or printing at low manufacturing costs. In addition, π-conjugated polymers have high glass transition temperatures, so that a thin layer having excellent mechanical properties can be obtained. Accordingly, π-conjugated polymer organic electroluminescent devices are expected to have a commercial competitive edge over low molecular weight electroluminescent devices.
Such π-conjugated polymer electroluminescent devices, however, have lower emissive luminance than low molecular weight electroluminescent devices and exhibit poor durability due to deterioration of the emissive polymer. During synthesis of such polymer materials, defects may be generated, resulting in the deterioration of molecular chains. Moreover, it is difficult to refine the synthesized polymer to a high degree. Developing a polymerizing technique that minimizes defects generated in the π-conjugated polymers may overcome these problems, as well as by developing a refining technique that can remove impurities existing in the π-conjugated polymers. If π-conjugated polymer materials synthesized in the above-described manner are continuously fed back to improve the performance of an organic electroluminescent device, an organic electroluminescent device with superior performance can be accomplished.
In addition, when the π-conjugated polymer material is used to manufacture a multi-layered thin film having various functions, which is usually formed using a low molecular weight molecule, stability of thin films, in particular, uniformity of the thin films is lower than when using a low molecular weight molecule, and also only limited solvents can be used. As a result, thin films cannot be easily formed using π-conjugated polymer material. Many efforts to solve these problems have been made. For example, like a low molecular weight electroluminescent device, a π-conjugated polymer electroluminescent device with high efficiency and a long lifetime can be obtained by using a multilayer system including, e.g., a buffer layer, a hole transporting layer, an electron transporting layer, and a hole blocking layer.
Many efforts have focused on manufacturing a stable multilayer thin film providing various functions. For example, the introduction of an intermediate layer after forming a hole transporting layer thin film and before depositing an electroluminescent layer on the intermediate layer, thereby obtaining high emission efficiency and a long lifetime has been proposed. Here, the introduction of the intermediate layer minimizes quenching of excitons and degradation of an electroluminescent layer. The quenching occurs when excess electrons, which result from unbalanced holes and electrons, combine with holes to form excitons which travel to the hole transporting layer.
Referring to FIG. 1, an intermediate layer having a larger band gap than the hole transporting layer and an electroluminescent layer including a hole transporting material within its structure is shown. In this case, the device has a properly controlled HOMO value. As a result, holes can travel into the emissive layer without experiencing a barrier. However, since the LUMO value is small, the intermediate layer functions as a barrier when excess electrons and excitons travel into the hole transporting layer. Indeed, the electrons and excitons carried to the hole transporting layer can be minimized, so that a recombination zone, where excitons are generated, exists only within the emissive layer. Therefore, an organic electroluminescent device with a higher efficiency and a longer lifetime can be manufactured.
Alternatively, depending on what substance was used to fabricate the intermediate layer, red, green, and blue luminescent devices will exhibit different characteristics from one another. In particular, a blue luminescent device having an intermediate layer exhibits enhanced efficiency, which can be more than two times the efficiency of an intermediate layer-free blue luminescent device. Moreover, the blue luminescent device may last for 10,000 hours or longer as disclosed in U.S. Pat. No. 5,858,562.
Since the intermediate layer is soluble in certain solvents, however, suitable solvents that may be used to the electroluminescent layer may be limited. As a result, the thickness of the thin films may not be easily controlled. Furthermore, in order to adjust the band gap of the intermediate layer and the LUMO/HOMO value according to the electroluminescent layer used, optimization of the structure of the polymer chains may be required. However, synthesis of the polymer is not easily performed and there is a limit to the adjustment.